The invention concerns a steam dryer, in particular a steam dryer intended for drying of concentrate materials, which dryer comprises a cylindrical, rotatable drum, in whose interior a set of steam pipes consisting of a number of pipes has been fitted to revolve along with the drum, into which set of steam pipes the steam intended for drying of the material has been arranged to be supplied by means of steam supply devices fitted at a first end of the dryer, and in which steam dryer, at the opposite end, a conveyor or equivalent has been installed to supply the material to be dried into the drum interior, from where the dried material has been arranged to be discharged through the first end of the dryer.
In the prior art, a number of different types of dryer drums based on the use of steam are known, which drums are used, among other things, for drying of different concentrate materials. In steam dryers of said type, the material to be dried is introduced into the drying drum, and the steam intended for drying is made to flow in pipes fitted inside the drum. Thus, the material to be dried cannot reach direct contact with the steam. Drying drums can be divided into two main types, i.e. into batch-type dryers and, on the other hand, into continuous dryers. In batch-type dryers, of which, as one example, the dryer described in the German Patent No. 27 24 639 can be mentioned, the drying drum is filled to the desired quantity, the supply of steam is started into the set of pipes placed in the interior of the drying drum, and at the same time the drying drum starts revolving. After the material to be dried has been made sufficiently dry, the rotation of the drum is stopped, the supply of steam is switched off, and the drum is emptied. Thus, it can be considered to be a drawback of a batch-type dryer that, exactly owing to its mode of operation, the dryer cannot be used smoothly for drying of large quantities of material.
In continuous dryers, the arrangement is, as a rule, such that the drying drum is rotated constantly, and material to be dried is supplied into the interior of the drum through one end of the drum. During drying, steam is passed constantly into the pipe system provided in the interior of the drum. The drum is provided with purposeful means by which it is rotated during drying, and further, the concentrate material that is fed into the drum during drying is carried towards the opposite end of the drum, out of which end the material is discharged. As a rule, the drum is provided with an adjustable overflow edge or with adjustable discharge openings, and further, it is common that the position of inclination of the drum can be adjusted within certain limits so as to regulate the dwell of the concentrate material in the drum. As an example of such a continuous drying drum, the steam dryer described in the applicant""s Patent Application No. 962853 of earlier date can be mentioned.
As regards the construction of prior-art steam drum dryers, it can be stated by way of example that quite commonly such dryers are used in which the steam pipes have been fitted as substantially parallel to the axis of the drum. In such an arrangement, in some cases, it has been noticed to be a problem that an insulating material layer is formed on the faces of the steam pipes out of the material to be dried. In such prior-art dryers in which longitudinal pipes are used, the filling degree of the devices is relatively low, i.e. of an order of 10 . . . 15% of the inside volume of the drum, and such drums are not used commonly in drying of concentrate materials because of their large size and low efficiency.
A further prior-art dryer arrangement is such that in it a separate rotor is employed, which forms the heating face and around which rotor there is a stationary fixed basin placed in a horizontal plane. As a rule, the rotor consists of a number of parallel groups of pipes, which have been arranged along a central tube. Each group of pipes usually comprises a number of concentric pipe rings, and the heat transfer medium that is used can be steam or a liquid. In this prior-art solution, the system of steam pipes, which has been mounted from its ends by means of bearings and which revolves in the concentrate to be dried, imposes a restriction of size, and the size of the equipment cannot be increased economically when the amount of concentrate is increased. A considerable drawback of dryers of this type is rapid wear of the steam pipes, which causes great problems in drying of abrading concentrates, because the rotor revolves in a stationary concentrate bed, in which case the pressure applied by the concentrate to the steam pipes is high and causes intensive wear, as was mentioned above.
The object of the present invention is to provide a steam dryer in particular intended for drying of concentrates, in which dryer abrasion has been eliminated or at least minimized or in which abrasion does not cause problems otherwise, and in which dryer no insulating layer of concentrate can be formed on the steam pipe faces.
Further, it is an object of the solution in accordance with the present invention to provide a steam dryer whose size is not limited, but which can be constructed in each case suitable for the purpose in compliance with the desired production quantity.
Further, it is an object of the present invention to provide a steam dryer in which it is possible to utilize steam that is produced elsewhere in the process of treatment of the concentrates. In view of achieving the objectives of the invention, the present invention is mainly characterized in that the set of steam pipes is composed of a number of pipe elements, each of which comprises two axial pipes parallel to the longitudinal direction of the drum and a number of pipe arcs fitted in the cross direction of the drum and at a distance from one another in relation to the longitudinal direction of the drum, which pipe arcs interconnect the two axial pipes of each pipe element, and that, at the first end of the steam dryer, a steam distributor header has been fitted coaxially with the drum, through which header the steam meant for drying has been arranged to be fed into each pipe element, and through which steam inlet header, in a corresponding way, the removal of the condensate coming from the pipe elements has been arranged.
In the arrangement of equipment in accordance with the invention, the set of steam pipes in the dryer revolves along with the drum, in which case no pressure and no abrading difference in speed is formed between the material to be dried and the steam pipes, and further, longitudinal raising blades fitted on the inside face of the drum eliminate excessive gliding of the material on the face of the drum, in which case abrasion of the steam pipes or formation of a material layer on the faces of the steam pipes do not cause problems, nor is the size of the construction restricted by the bearing system of the steam pipes or by any other, similar factor. In a steam dryer in accordance with the invention, the concentrate to be dried is supplied into the drum through one end of the drum by means of a suitable conveyor, and the concentrate is discharged out of the drum through discharge openings provided at the opposite end of the drum. The dryer is provided with adjustable discharge openings, through which the dried material is discharged and by means of which discharge openings an adequate dwell of the material in the dryer and, at the same time, a suitable filling degree are secured.
The drum of the dryer and the set of pipes placed inside the drum are rotated during drying, and therefore the drum is provided with a purposeful mechanism of rotation, for example, similar to that described earlier in the applicant""s Finnish Patent Application No. 962853. Also, a mechanism of rotation of a different sort suitable for the purpose can be used.
The set of pipes placed in the drum has been formed and composed of elements, which can be removed from the drum and replaced separately, whereas, in the prior art, in the applicant""s earlier Finnish Patent Application No. 962853, the set of steam pipes consisted of one large and heavy part. In this novel dryer, the service life and the usability of the dryer can be improved, because, if necessary, the tube elements can be replaced quickly and easily, which reduces the standstill time of the dryer.
By means of the steam dryer in accordance with the invention, high drying efficiency is obtained, because the filling degree of the drum can be made very high and the drum is filled with steam pipes. In a dryer in accordance with the invention, a filling degree of even higher than 30 per cent can be achieved.
The drum of the steam dryer in accordance with the invention can also be in an inclined position so as to enhance the transfer of the material to be dried and to remove the condensate.